1. Field of the Invention
The present invention relates to photo induced-electromotive force (EMF) detectors. More particularly, the present invention relates to electrode design and processing, which shields the depletion region adjacent to the electrodes from optical radiation.
2. Description of Related Art
Conventional photodetectors typically generate a signal that is proportional to the power of an incident beam of light. Recently, a new photodetector has been developed based on non-steady-state photo-electromotive force (photo-EMF). A photo-EMF detector generates an output signal proportional to the transient lateral motion of an incident optical pattern on its surface. When the pattern is stationary, no output signal is produced, regardless of the location or amount of incident power.
A conventional photo-EMF sensor resembles a conventional semiconductor detector; however, it develops an internal lateral electric field that stores the spatial intensity pattern of an incident optical beam. FIG. 1a shows a conventional photo-EMF sensor 100 comprising a detection substrate 110 and a pair of surface electrodes 120. FIG. 1b also shows the intensity pattern 150 of an incident laser beam 152 while FIG. 1c depicts the resulting lateral internal space charge field 160 developed in the material between the electrodes 120 shown in FIG. 1a. The detection substrate 110 may comprise gallium arsenide and the surface electrodes 120 may comprise a titanium/gold alloy.
A photo induced-EMF effect is generated when two coherent optical beams are directed to interfere at the detector. Typically, one of the beams (the probe) contains both temporal (desirable information) and spatial phase content. The second beam (a reference beam) is usually a plane wave or a wavefront with a smoothly varying phase and is typically relatively constant in time and space. At the detector surface the probe beam interferes with the reference beam producing an interference fringe pattern. Photo-generated carriers are then produced which diffuse away from regions of intense optical radiation. These carriers become trapped and form a periodic charge pattern and the corresponding space-charge field. At steady state, (i.e., in the absence of any change to the interference pattern) the space charge field is static and there is no net current between the electrodes. However, when the phase of the probe beam is modulated rapidly (relative to the space-charge formation time), the interference fringes move relative to the initial stationary space charge field grating and induce a net current or photo-EMF. For phase changes comparable to the response time of the detector material, the space charge gratings track the motion of the intensity pattern across the material, resulting in no new current being produced. Therefore, the output current as a function of time from the detector is proportional to the phase modulation. The frequency response may be as high as several hundred MHz to GHz and is constrained by the carrier recombination time of the material, the grating fringe spacing and signal-to-noise considerations.
A photo-EMF detector is commonly used to measure ultrasonic waves in materials, just as a conventional contact piezoelectric ultrasonic transducer (PZT) does, but without contacting the sample under inspection. The special properties of a photo-EMF detector permit in-process ultrasonic inspection in industrial environments where background vibrations, rough surfaces, high temperatures, and rapid process motions prevent contact PZTs and traditional laser interferometers from being used.
In addition, a photo-EMF detector may also be used in optical communication for free-space links, optical fiber busses, which may be used over terrestrial and satellite networks. Further, a photo-EMF detector may be used in optical remote sensing applications, where phase-encoded information is to be detected, or in laser based process control including composite cure monitoring and quality assurance.
In a laser-ultrasound probing system, the dynamic phase shift may be the result of motion in a probed surface. In a laser communication receiver, the motion may be due to phase modulation encoded onto a light beam. The photo-EMF sensor is adaptive, since static and slowly varying changes, such as those due to beam wander, vibrations, thermal effects, turbulence, are adaptively tracked. The photo-EMF sensor will produce no current for such changes, as long as the space-charge field formation time is faster than the changes. Thus, the photo-EMF sensor can adaptively compensate for the effects due to these changes. This adaptive compensation capability makes the photo-EMF sensor extremely attractive for many applications, since the sensor detects high frequency information, while suppressing low frequency noise.
In prior art photo-EMF systems, continuous wave (CW) light incident on the detector's sensitive surface near the electrodes produces a photocurrent that contributes to the noise. This noise current is additive to the desired time modulated signal and directly reduces the signal-to-noise ratio. This current can be a particularly significant source of noise if the illumination energy is above the bandgap because most of the light is absorbed very close to the surface (˜μm). This implies that the depth of the surface depletion region arising from Fermi level pinning can be on the same order as the absorption depth for photon energies above the band gap. The effect of photocurrent generated near the electrode can be divided into two distinct cases, continuous wave (CW) and temporally modulated illumination. In the first case, CW light incident on the depletion region near the electrodes produces a DC current. This current is converted into a voltage by a transimpedance amplifier and further amplified by additional gain stages. When the unwanted DC offset is amplified by subsequent gain stages the final stages of amplification can become saturated or the dynamic range of the amplifier can be severely restricted. When the amplifier chain performance is degraded in this manner it directly affects the dynamic range of the sensor.
In the second case, the performance of the sensor can be impaired because of amplitude modulation (AM) illumination from one of the beams incident on the depletion region. This AM light produces a temporally modulated current that cannot be distinguished from the desired signal produced by the phase modulation of the probe beam. Because the currents are additive, the AM noise is added to the signal and therefore reduces the overall sensor signal-to-noise ratio. The AM modulation can be due to speckle effects, beam dropouts or severe beam wander; in all cases, this modulation is undesirable.
Therefore, there exists a need in the art for a method and apparatus that will reduce the noise generated near the electrodes of a photo-EMF sensor.